In copiers and printing machines, in particular in electro-photographic printing machines, toner from inking devices is applied to a printing medium for the purpose of generating on the printing medium an ultimate image that corresponds to the data provided for the desired image. In order to avoid smearing of the toner on the printing medium the toner is generally fused by simultaneously applying pressure and heat to the surface of the printing medium.
For this purpose fuser mechanisms are used, which can contain the various fuser elements. In most cases these mechanisms contain a fuser roller and a pressure roller that is located across a printing medium transport path from the fuser roller. The path traveled by the printing medium leads between the fuser roller and the pressure roller through the so-called nip. The fuser roller is heated for the fusing process and the pressure roller is pressed against the fuser roller. This enables the toner to melt and ultimately to fuse onto the surface of the printing medium. It is also possible that the two fusing elements, i.e., the fuser roller and the pressure roller, are identically formed and that both are heated.
To allow the printing medium to pass smoothly through the nip, the fuser roller and the pressure roller each rotate in the printing medium's direction of travel. A problem arises in this process at the moment when the printing medium on which the toner layer has been fused is supposed to separate from the fuser roller. At that moment toner offset can occur, whereby same toner detaches itself from the printing medium and becomes attached to the surface of the fuser roller. The resulting image is then severely adversely affected and the fuser roller becomes contaminated.
Separating agents are used to solve the problem. Separating agents characteristically reduce the bond between the surface of the toner and the fuser roller, so that such bond is out-weighed by the bond between the toner and the printing medium. Accordingly, offset is substantially prevented. Silicon oil is the main choice among the separating agents in use. It is applied to the surface of the fuser roller on the upstream side of the nip. For this purpose coating rollers, for example, are used to apply the silicon oil.
Once the separating agent is on the fuser roller, it must be sufficiently viscous so that it remains on the surface of the fuser roller during the fusing process and does not become transferred to the printing medium. During his time the viscosity of the separating agent is a function of the temperature. Thus, if the silicon oil must be maintained at a certain viscosity on the surface of the heated fuser roller, its viscosity at room temperature must be significantly higher. Because of the resultant high viscosity of the “cold” separating agent, the problem arises that the separating agent does not spread out homogeneously in a thin layer, for example, when being transferred from the coating roller to the surface of the fuser roller. The result is that the distribution of the separating agent on the surface is non-homogeneous so that streaks appear on the image gloss.
What proves to be an even more persistent and unpleasant problem arises, however, when surplus separating agent makes its way into the interior of the printing machine and then reaches the inking device. For example, when duplex printing is in progress, the upper sides of the printing medium, on which the separating agent is present, lie on the surface of whatever conveying medium is being used. This conveyor can, for example, be a belt. At least some of the separating agent can remain on this conveyor belt and can then contaminate the bottom side of subsequently conveyed printing media, or can even go directly into the inking devices.
The pressure roller, too, can become covered with separating agent by contact with the fuser roller. Here, too, the bottom side of the printing media can thus become contaminated. If the bottom side of printing media is contaminated with separating agent, what can often happen in accordance with what has been said above, is that in the course of duplex printing the bottom side of printing medium comes into contact with the inking devices, and then the inking devices become contaminated.
Once separating agent is in or on an inking device, the transfer characteristics of the inking device change. Depending upon the amount of the separating agent that is present, varying changes in the rate of toner transfer onto the printing medium occur. This can also occur as soon as separating agent is present in the area between the printing medium and the inking device. For such changes to occur, it is not absolutely necessary that the separating agent get into the inking device. It has been shown that the dependence of the transfer characteristic of the inking device upon the amount of the separating agent present in this area is, at least with respect to the use of the standardly used silicon oils, not linear. As the silicon oil begins to enter the inking device, the amount of toner transferred increases at first, and then as more oil enters the inking device a maximum is reached, which is then exceeded. In any case, the amount of toner always deviates from the desired amount, and the deviation differs from place to place in the inking device, depending upon the amount of the silicon oil present. But even if the transfer characteristics of the inking devices vary linearly as a function of the amount of separating agent present, highly noticeable changes in the amount of toner on the printing medium as a function of the separating agent present occur.
In duplex printing, a non-homogeneous distribution of silicon oil on the surface of the printing medium can exist when the second side is printed. This non-homogeneous distribution of silicon oil is based mainly on the fact that the viscosity of the silicon oil is very high at room temperature, and an even application of silicon oil with this viscosity is essentially impossible. This lack of homogeneity leads to the inking devices having varying transfer characteristics as they transfer toner onto the surface of the printing medium, and thus to the resulting image, displaying noticeable streaks.
Aside from the fact that a complete and even application of separating agent onto the surface of fuser roller is not possible, in all cases at least enough separating agent must be transferred to the surface so that the printing medium, across its entire width and together with the total toner layer, separates from the fuser roller without offset. Thus, at least a minimal amount of separating agent is always necessary. Because of the non-homogeneous application of the separating agent there are always areas of on the surface of the printing medium that are contaminated with the separating agent. This permits separating agent to be carried into the printing machine.
One way of preventing separating agent from being carried into the printing machine is to free at least the surface of the pressure roller from residual separating agent. For this purpose, it is suggested, for example, that blades be used for scraping the separating agent off the surface of the pressure roller. Of course, one is confronted here, too, with conflicting interests. That is to say, one may want to leave at least a small amount of separating agent on this surface so that the printing medium will separate from the pressure roller with the greatest possible ease. In addition, when a blade is used, it is not possible to completely clean off the pressure roller. There will always be a residue of separating agent on the surface of the pressure roller, which can then reach the interior of the printing machine by way of a printing medium.